1. Field of the Invention
The present invention relates to a scanning fluorescent microscope that irradiates a sample with exciting light pulses, counts the number of fluorescent photons emitted from the excited sample, and measures at least a fluorescent lifetime based on the counted number of the fluorescent photons.
2. Description of the Related Art
To examine the excited state of a sample, a conventionally known technique includes irradiating the sample with exciting light to turn the sample into an excited state, measuring the fluorescence emitted from the sample during the transition from the excited state to the ground state, and calculating the fluorescent lifetime. One known method of calculating a fluorescent lifetime is a Time-Gate Technique, according to which a sample is irradiated with exciting light pulses, the number of fluorescent photons emitted from the sample is measured at plural periods of time, and the fluorescent lifetime is calculated based on the measured numbers of fluorescent photons (see C. J. DE GRAUW and H. C. GERRITSEN, “Multiple Time-Gate Module for Fluorescence Lifetime Imaging”, APPLIED SPECTROSCOPY, Volume 55, Number 6, 2001).
In order to minimize the error in fluorescent lifetime calculation, measurement should be conducted on a large number of fluorescent photons. However, an increase in the emission rate of fluorescent photons with an increase in the intensity of the exciting light results in an increase in the measurement error of the lifetime. This is because that the fluorescent photons successively entering a measurement device cannot be captured within the measurement resolution thereof. Hence, for the minimization of the error in fluorescent lifetime calculation, the number of emitted fluorescent photons per one irradiation of exciting light (emission rate) needs to be equal to or less than 0.01. In other words, calculation of fluorescent lifetime is extremely time-consuming when error minimization is required. In addition, in calculation of the fluorescent lifetime of a living body, for example, in which the fluorescent lifetime changes over time, the speed of calculation cannot outpace the speed of changes over time, whereby the observation of fluorescent lifetime distribution in a living body or the like is virtually impossible.